The present invention relates to water-based drilling fluids used during petroleum recovery operations, and more particularly relates, in one embodiment, to using water-based drilling fluids containing additives to inhibit penetration of the borehole wall by the fluid.
Drilling fluids used in the drilling of subterranean oil and gas wells as well as other drilling fluid applications and drilling procedures are known. In rotary drilling there are a variety of functions and characteristics that are expected of drilling fluids, also known as drilling muds, or simply xe2x80x9cmudsxe2x80x9d. The drilling fluid is expected to carry cuttings up from beneath the bit, transport them up the annulus, and allow their separation at the surface while at the same time the rotary bit is cooled and cleaned. A drilling mud is also intended to reduce friction between the drill string and the sides of the hole while maintaining the stability of uncased sections of the borehole. The drilling fluid is formulated to prevent unwanted influxes of formation fluids from permeable rocks penetrated and also often to form a thin, low permeability filter cake which temporarily seals pores, other openings and formations penetrated by the bit. The drilling fluid may also be used to collect and interpret information available from drill cuttings, cores and electrical logs. It will be appreciated that within the scope of the claimed invention herein, the term xe2x80x9cdrilling fluidxe2x80x9d also encompasses xe2x80x9cdrill-in fluidsxe2x80x9d.
Drilling fluids are typically classified according to their base material. In water-based muds, solid particles are suspended in water or brine. Oil can be emulsified in the water or brine. Nonetheless, the water is the continuous phase. Oil-based muds are the opposite. Solid particles are suspended in oil and water or brine is emulsified in the oil and therefore the oil is the continuous phase. Oil-based muds that are water-in-oil emulsions are also called invert emulsions. Brine-based drilling fluids, of course are a water-based mud in which the aqueous component is brine.
Optimizing high performance water base mud design is commonly at the forefront of many drilling fluid service and oil operating companies"" needs due to the various limitations of invert emulsion fluids. Invert emulsion fluids formulated with traditional diesel, mineral or the newer synthetic oils are the highest performing drilling fluids with regard to shale inhibition, borehole stability, and lubricity. Various limitations of these fluids, however, such as environmental concerns, economics, lost circulation tendencies, kick detection, and geologic evaluation concerns maintains a strong market for high performance water based fluids. Increased environmental concerns and liabilities continue to create an industry need for water based drilling fluids to supplement or replace the performance leading invert emulsion mud performance.
A particular problem when drilling into shale formations with water-based fluids is the pore pressure increase and swelling from penetration of the shale by the fluid. Shale stabilizers are typically added to the mud to inhibit these phenomena and to stabilize the shale from being affected by the mud.
Reducing drilling fluid pressure invasion into the wall of a borehole is one of the most important factors in maintaining wellbore stability. It is recognized that sufficient borehole pressure will stabilize shales to maintain the integrity of the borehole. When mud or liquid invades the shale, the pressure in the pores rises and the pressure differential between the mud column and the shale falls. With the drop in differential pressure, the shale is no longer supported and can easily break off and fall into the well bore. Likewise, the invasion of water into the shale matrix increases hydration or wetting of the partially dehydrated shale body causing it to soften and to lose its structural strength. Chemical reactivity can also lead to instability. There is always a need for a better composition and method to stabilize the shale formations.
It is apparent to those selecting or using a drilling fluid for oil and/or gas exploration that an essential component of a selected fluid is that it be properly balanced to achieve all of the necessary characteristics for the specific end application. Because the drilling fluids are called upon to do a number of tasks simultaneously, this desirable balance is difficult to achieve.
It would be desirable if compositions and methods could be devised to aid and improve the ability of drilling fluids to simultaneously accomplish these tasks.
Accordingly, it is an object of the present invention to provide methods to stabilize shale formations when drilling with water-based drilling fluids.
It is another object of the present invention to provide water-based drilling fluids that reduce the rate of drilling fluid pressure invasion into the borehole wall.
Still another object of the invention is to provide a composition and method that increase the pressure blockage, reliability, magnitude, and pore size that can be blocked with water-based fluids for stabilizing shale formations.
In carrying out these and other objects of the invention, there is provided, in one form, a water-based drilling fluid including water and a polymer latex capable of providing a deformable latex film or seal on at least a portion of a subterranean formation.